Blow molding is a common process for forming plastic bottles and similar containers. The blow molding process typically includes heating and pressurizing the interior of a plastic preform inside a mold to expand the preform to the shape of the mold. Preforms are often formed of a thermoplastic, for example HDPE or PET, by injection molding, extrusion, or a combination of both methods.
Blow molding processes include injection blow molding and extrusion blow molding, each of which generally correspond to the method of forming the preform. The injection blow molding process typically conveys, mixes, melts, and injects resin into an injection mold, often using a screw conveyor. The extrusion blow molding process may be continuous, in which molds move around the preform and then move away from the extruder, or intermittent in which the extruder intermittently pushes the extrusion in a mold similar to injection blow molding.
A typical preform, of the type that may be blown into a plastic bottle, includes an open end, a substantially cylindrical body, and a rounded tip at a closed end. The open end may have screw threads formed near its neck, and a neck support ring, which forms the largest diameter of the preform. Between the cylindrical body and the rounded tip is a base transition line. At the end of the rounded tip is a portion of plastic termed a gate.
Preform design and production quality is crucial to forming a blow-molded container having sound physical properties. Further, because each final container design requires its own unique preform shape, quality control measures often must be modified to accommodate each preform. Preform wall thickness tolerances are crucial to proper bottle production. Specifically, concentricity, which is the degree to which wall thickness varies around the circumference of a section of the preform, is an important characteristic. For example, in injection molding, a predetermined volume of plastic is utilized for the preform. High concentricity may indicate that one part of the preform wall has too much of the plastic, and another part of the preform wall may have insufficient thickness to form an adequate container wall, or even that the wall may be formed with voids. Measuring the wall thickness of a preform is an important quality and inspection function. Instruments are employed to measure preform wall thickness. For example, a measuring device may have a magnetic sensor tip that measures the distance from the tip to a magnetic ball according to the Hall effect of the ball on the magnetic field. An example of such a measuring device is the MAGNA-MIKE(.TM.) Model 8000 measuring device, available from Panametrics of Waltham, Mass.